The introduction of the ULEZ in 2019 has lead to this great decline in fuel purchasing which has called for other cities to also introduce similar zones. Reductions in the usage of diesel fuel in vehicles will have significant impacts on the improvement of overall air quality in London and thus respiratory health.
This article explains more: https://airqualitynews.com/cars-freight-transport/sale-of-diesel-in-london-falls-40-in-four-years/